This invention relates generally to fire-safe, non-rising stem type gate valves and, more specifically, to such a fire-safe gate valve having a fusible body or bodies which upon melting permit the stem to rise relative to the bonnet to provide a metal-to-metal backseat therebetween.
Gate valves, both slab type and expanding type, are used in the pipeline industry to control fluid flow in oil and gas pipelines. Such valves are provided with packing assemblies which work well in maintaining the integrity of the valve under ordinary circumstances, but which are subject to being damaged should there be a fire, the heat of which engulfs the valve for more than a short period of time. Accordingly, gate valves have been constructed which provide a metal-to-metal backseat seal between the stem and valve bonnet which are normally kept in a spaced relationship to one another by using one or more fusible bodies. If a fire occurs subjecting the valve to sufficient heat to melt the fusible materials, the valve components are permitted to shift allowing the metal-to-metal backseat to complete a seal. Examples of gate valves utilizing fusible materials to provide a backseat are showin in the following table of U.S. patents:
______________________________________ Patentee U.S. Pat. No. Issue Date ______________________________________ Wicke 3,771,540 Nov. 13, 1973 Allen 3,788,600 Jan. 29, 1974 Wicke 3,842,854 Oct. 22, 1974 Wicke 3,896,835 Jul. 29, 1975 Williams, Jr. et al 4,214,600 Jul. 29, 1980 McGee 4,245,661 Jan. 20, 1981 McGee 4,289,157 Sep. 15, 1981 ______________________________________
However, in most of the above U.S. patents the fusible materials are positioned above the stem bearings relative to the flow passage of the valve. In this position when the fusible material melts the bearings and valve stem will be forced upwardly or outwardly from the flow passage by the pressurized fluid existing therein. Thus, the stem is allowed to backseat against the valve bonnet to provide sealing protection for the packing assembly and upper bonnet areas of the valve. However, since the bearings are allowed to move with the stem there exists the possibility that the bearings will become cocked or wedged against the stem, thereby making it difficult or impossible to cycle the valve after the fire has extinguished.
Furthermore, in the prior art patents listed above, the valve stem and gate member are connected so that when the fusible material melts and vacates its cavity the gate member will shift with the valve stem to provide the backseat seal with the valve bonnet. However, pressurized fluid in the upstream flowline continually acts on the gate member when it is in the closed position to create a drag force on the face thereof which impedes any upwardly or outwardly movement of the gate member. Accordingly, the valve stem can be prevented from backseat sealing with the bonnet assembly due to its connection with the gate member and, thus, the packing assembly will be subjected to the highly pressurized fluid existing within the valve during a fire condition. Additionally, the packing assembly may deteriorate due to the extreme heat caused thereby and flowline fluid will then leak exteriorly of the valve to further fuel the fire condition occurring exteriorly of the valve. Therefore, it is of beneficial value to have a gate valve with a heat responsive backseat which allows the valve stem to backseat with the bonnet assembly independently of the gate member.